Astronomical basis of Vedic Astrology

The Zodiac is a belt of heavens or the physical sky that you can I see up above us, extending on both sides of the ecliptic. It encircles the space through which the planets travel in their
own defined orbits.

Vedic Astrology employs sidereal zodiac, which is different from the tropical zodiac used in Western Astrology. While the tropical (Western) Astrology uses the vernal equinox (the Sun’s
position at the beginning of spring) as the starting point for the measurements along the zodiac, the sidereal (Vedic) Astrology uses fixed stars to identify different segments along the zodiac.

The starting points of the sidereal and tropical zodiacs coincide once every 25,800 years. After that the starting points separate from each other by an approximate 1 degree of an arc per each 72
years. The difference between the longitude of the starting points of the sidereal and tropical zodiacs at any given time is called Ayanamsha.

Due to the existing controversy about the year in which the two starting points coincided last, there are several ayanamshas used by different schools of Vedic astrologers. Some of them are
Lahiri, Krishnamurti, Raman, and Fagan ayanamshas. Lahiri is the most widely used Ayanamsha which is based on the last coincidence point taking place in the year 285 A.D.

Converstions from Siderial Zodiac to Tropical Zodiac and Vice Versa

Lahiri Ayanamsha for the year of your birth is 00:00:16 degrees. If you want to convert your sidereal (Vedic) planetary positions into tropical (Western), you can do so by adding this
ayanamsha to the degrees of the planets in your Vedic chart. To arrive at the sidereal positions by converting the tropical ones, you will need to subtract the ayanamsha from the tropical
positions.

For example, your Sun is placed in 09:53 degrees of the constellation of Leo in your Vedic chart. When you add the ayanamsha for the year of your birth, which is 00:00:16 degrees, you
will derive the tropical (Western Astrology) horoscope position of your Sun which happens to be placed in the sign of Leo. Therefore, your Jyotish (sidereal) Sun sign is Leo and your
Western (tropical) Sun sign is Leo.